axis, xlab, labels, data.frame drop, etc
axis, xlab, labels, data.frame drop, etc

- new: axis.args
- new: return value breaks "at" and "label" down by factor/subfactor
- new: xlab for each factor label
- changed: do not try to plot densities if width is 0
- changed: groups is SIGNIFICANTLY DIFFERENT (better)
- changed: modify grid (abd abline) to intercept grid(nx) and draw
  vertical lines at each density plot
- changed: split sep from default (".") to "|"
- fixed: data.frame subsetting was dropping to a vector if ncol 1
- fixed: lines were not counting correctly
- fixed: order is preserved in the output lists
- fixed: xlim not influenced by boxplot boxwex

r2evans/violinplot